Additive manufacturing of ceramic materials is of interest for propulsion components, thermal protection systems, porous burners, microelectromechanical systems, and electronic device packaging, for example. Ceramic structures are typically sintered as compacted porous materials, severely decreasing the overall strength of the material. Thus, there exists a need for creating large, fully dense ceramic materials which possess the high strength of the parent material and are therefore useful for engineering applications.
Formulations have been described for creating ceramic materials, which can be printed (additively manufactured) with various methods such as stereolithography techniques and laser sintering. These are typically sintered powders or formulations with solid material suspended, typically producing porous structures. These methods are described in Zocca et al., “Additive Manufacturing of Ceramics: Issues, Potentialities, and Opportunities”, J. Am. Ceram. Soc., 98 [7] 1983-2001 (2015).
In addition, formulations which can create 1D or 2D ceramics, or very small 3D structures, have been described. See U.S. Pat. No. 4,816,497 issued Mar. 28, 1989 to Lutz et al.; U.S. Pat. No. 5,698,485 issued Dec. 16, 1997 to Bruck et al.; U.S. Pat. No. 6,573,020 issued Jun. 3, 2003 to Hanemann et al.; U.S. Pat. No. 7,582,685 issued Sep. 1, 2009 to Arney et al.; and U.S. Patent App. Pub. No. US2006/0069176A1 published Mar. 30, 2006 to Bowman et al.
In comparison with metals and polymers, ceramics are difficult to process, particularly into complex shapes. Because they cannot be cast or machined easily, ceramics are typically consolidated from powders by sintering or deposited in thin films. Flaws, such as porosity and inhomogeneity introduced during processing, govern the strength because they initiate cracks, and—in contrast to metals—brittle ceramics have little ability to resist fracture. This processing challenge has limited the ability to take advantage of ceramics' impressive properties, including high-temperature capability, environmental resistance, and high strength. Recent advances in additive manufacturing have led to a multitude of different techniques, but all additive manufacturing techniques developed for ceramic materials are powder-based layer-by-layer processes. Only a few of the commercially available three-dimensional (3D) printing systems offer printing of ceramics, either by selective curing of a photosensitive resin that contains ceramic particles, selective deposition of a liquid binder agent onto ceramic particles (binder jetting), or selective fusion of a powder bed with a laser. All these techniques are limited by slow fabrication rates, and in many cases, a time-consuming binder removal process. By starting with powders, consolidation to a dense part is an almost insurmountable challenge, and residual porosity is typically unavoidable. Furthermore, many additive processes introduce large thermal gradients that tend to cause cracks in ceramics. Pores, cracks, and inhomogeneities are responsible for the low strength and poor reliability of additively manufactured ceramic parts.
Preceramic polymers are a class of polymers which allow, via a thermal treatment, a conversion of a polymer part to a ceramic material. Typically, these preceramic polymers contain silicon (Si) in the molecular backbone, with the resulting material containing Si. There are a wide variety of known preceramic polymers. Examples include polysilazanes, borasine-modified hydridopolysilazanes, polysilanes, polycarbosilanes, silicone resins, polyvinylborazine, polyborazylene, and decaborane-based polymers. These preceramic polymers have been used to form specific polymer-based structures that can be subsequently heat-treated (pyrolyzed or sintered) to create near net-shape ceramic structures.
A stereolithography technique provides a method to build a 3D polymer microstructure in a layer-by-layer process. This process usually involves a platform (e.g., substrate) that is lowered into a photomonomer bath in discrete steps. At each layer, a laser is used to scan over the area of the photomonomer that is to be cured (i.e., polymerized) for that particular layer. Once the layer is cured, the platform is lowered by a specific amount, determined by the processing parameters and desired feature/surface resolution, and the process is repeated until the complete 3D structure is created. One example of such a stereolithography technique is disclosed in U.S. Pat. No. 4,575,330 issued Mar. 11, 1986 to Hull et al.
Modifications to the above-described stereolithography technique have been developed to improve the polymer resolution by using laser optics and special resin formulations. Also, modifications have been made to decrease the fabrication time of the 3D polymer structure by using a dynamic pattern generator to cure an entire layer at once. One example of such a modification is disclosed in Bertsch et al., “Microstereolithography: A Review,” Materials Research Society Symposium Proceedings, Vol. 758, 2003. Another advancement to the standard stereolithography technique includes a two-photon polymerization process, as disclosed in Sun et al., “Two-Photon Polymerization And 3D Lithographic Microfabrication,” Advances in Polymer Science, Vol. 170, 169-273, 2004.
However, no prior art teaches the formation of fully dense, high-strength, polymer-derived ceramic parts through UV-cure-based 3D printing or stereolithography, from Si-containing or related resin formulations. Current state of the art relies either on the sintering of ceramic particles or using ceramic particles printed in a binder, both of which produce porous ceramics. Porous ceramic structures have significantly lower strength than the parent material.
Direct, free-form 3D printing of preceramic polymers, which can be converted to fully dense ceramics, is sought. What are needed are low-cost structures that are lightweight, strong, and stiff, but stable in the presence of a high-temperature oxidizing environment. The monomers and polymeric systems preferably maintain properties so that they can be printed using stereolithography into complex 3D shapes. Ideally, the polymeric systems may be directly converted to fully dense ceramics with properties that approach the theoretical maximum strength of the base materials.